Steam-heated and induction-heated rollers are used in the paper making, printing, paper, film, and foil converting industries. Some examples are: web heating rollers, drying rollers and drums, laminating rollers, embossing rollers, and cast film extrusion rollers.
Steam-heated rollers are actually pressure vessels, especially at higher temperatures. The internal construction of both steam-heated and induction-heated cores can be quite complex and expensive in order to provide the temperature uniformity needed. In addition, a considerable amount of auxiliary equipment is needed to power or heat the roller.
Heat pipe technology has been used to control heat in various kinds of equipment used in space, remote monitoring stations and wherever heat transfer is required. A basic industrial heat pipe roller is disclosed in Noren, U.S. Pat. No. 3,700,028. As reported in Noren, "How Heat Pipes Work," Chemical Engineering, Aug. 19, 1974, acceptance of heat pipes in industry has been slow. Since that time a number of heat pipe constructions have been patented, often for small rollers used in office copiers and printers. Progress has remained slow, however, for industrial and large equipment applications.
International Publication Nr. WO 98/31194, published Jul. 16, 1998, discloses a heat pipe roller having an annular cavity between an inner cylindrical core and an outer cylindrical shell. To allow evacuation of the cavity and injection of an evaporative medium into the cavity, a charging port with a threaded plug is provided in one end of the roller. In the prior art, brazing or solder was used to seal the port after evacuation of air from the cavity and flowing of the medium into the cavity.
Prior art heat pipes are also of the gun-drilled type, in which one or more elongated heat pipe tubes are inserted in longitudinally extending gun-drilled holes in a roller core.
A primary cause of failure in heat pipes is the formation of non-condensable gases. Only one percent air or other gas in the heat pipe reduces heat pipe efficiency approximately fifty percent. Non-condensable gases can form as a result of corrosion, contamination, loss of vacuum, reaction of the medium with wicking materials, or degradation of the medium.
The maintenance of the seal on a charging port is important in preventing non-condensable gases. Such a seal should be easily assembled, and yet withstand internal pressures experienced by the heat pipe. The seal must be compatible with a charging sequence in which the heat pipe cavity is evacuated of air, the medium is flowed into the cavity and the port is sealed.